Acute leukemia is a malignancy of the hematopoietic elements that results at least in part from inappropriate activation of tyrosine kinases (TK). The most frequent somatic mutation associated with adult acute myeloid leukemia (AML) to date is the internal tandem duplication (ITD) of the FLT3 gene, a member of the Type III PDGF superfamily of receptor TKs. The FLT3 ITD defect results in the constitutive activation of the tyrosine kinase in the absence of ligand binding. Clinical studies thus far, however, have provided contradictory results with regards to presence of FLT3 ITD and prognostic significance of this defect in AML. These inconsistencies may be due to factors known to have confounding prognostic importance, such as varying cytogenetics, age, and treatment regimens. We examined a group of AML patients homogeneous for age, cytogenetics and treatment, and all considered at standard risk for relapse following therapy. We demonstrated three distinct genotypes among 82 patient samples examined: patients homozygous for the wild type (WT) FLT3 gene; patients heterozygous (FLT3ITD/WT), and patients hemizygous, i.e., FLT3 ITD in the absence of the WT gene, or FLT3ITD/-. Only the latter was a highly significant predictor of profoundly worse prognosis in AML patients compared to the others considered at standard risk. The overall research objective outlined in this proposal is to understand the mechanism by which the hemizygous genotype confers an especially poor prognosis, and to target this molecular defect in vitro and in vivo with a FLT3-specific inhibitor. To accomplish this goal, AIM 1 will investigate if a constitutively active mutant FLT3 in the absence of wild-type FLT3 confers a dominant positive gain-of-function role using in vitro and in vivo models. AIM 2 will assess whether proliferation and survival of FLT3 ITD-positive patient AML cells are selectively inhibited via induction of apoptosis by newly developed FLT3 inhibitor compounds. Funding of this K01 Mentored Minority Career Development Award will provide invaluable training for the applicant in the area of molecular mechanisms of disease, animal models for the study of human leukemia, and molecular targeted approaches to cancer therapy.